Ionomeric silane coupling agents

ABSTRACT

A novel ionomeric silane coupling agent is disclosed and its use in bonding a matrix polymer to a mineral substrate is described. The coupling agent is prepared by partially neutralizing the combined acid functionality present in a mixture of an acid-functional silane (or metal salt thereof) and an acid-functional film former with a metal ion. The coupling agent greatly improves bond strength in moist environments and is particularly suitable for use in formulating filled injection molding compositions.

This is a divisional of co-pending application Ser. No. 07/202,163 filedon 6-03-88 U.S. Pat. No. 4,863,978.

This invention relates to the field of silane coupling agents. Morespecifically, this invention relates to an ionomeric silane couplingagent prepared by partially neutralizing the combined acid functionalitypresent in a mixture of an acid-functional silane and an acid-functionalfilm former with a metal ion. It further relates to a process of usingthe ionomeric silane as a coupling agent to promote bonding between athermoplastic matrix polymer and a mineral or metal substrate.

BACKGROUND OF THE INVENTION

Silane coupling agents have been known to improve the mechanicalproperties of filled thermoseting and thermoplastic resins since thelate 1940's. These low molecular weight compounds are believed to formchemical links between filler particles and polymer molecules and assuch, they must incorporate functional groups capable of reacting, or atleast associating, with filler and resin alike. Although use of varioussilanes known in the art does greatly promote adhesion betweenthermoplastic polymers and substrates such as mineral fillers, exposureof these composites to water severely limits retention of the improvedadhesion. Thus, for example, a moist environment can induce a gradualdeterioration of the flexural strength of composites filled withsilane-treated reinforcing fibers, and there is still need forimprovement. Furthermore, when such fiber filled polymers are subjectedto high shear rates, as in an injection molding operation, there is atendency to destroy some of the covalent bonding (or any associativestructure) formed between the coupling agent and the polymer. This alsodetracts from ultimate physical properties of the composite. There isthus a need for a coupling agent which forms strong bonds orassociations between itself and the polymer under ordinary conditions,which bonds become highly mobile at the elevated temperatures and shearrates encountered during injection molding. Even more desirable would bethe availability of such a silane coupling agent which additionallyimparted bond durability when challenged by conditions of high moisture.

SUMMARY OF THE INVENTION

It has now been found that above mentioned desirable features can beachieved by treating a mineral substrate with an ionomeric silanecomposition comprising a mixture of an acid-functional silane and anacid-functional film former in which at least some of the combined acidfunctionality has been neutralized by the metal cation of an ioniccompound. One aspect of this concept is disclosed in a copendingapplication Ser. No. 202,164, filed on June 3, 1988, now U.S. Pat. No.4,871,788 hereby incorporated by reference.

Although not wishing to be bound by any particular theory orexplanation, it is believed that one end of the acid-functional silaneforms covalent bonds on the surface of the mineral substrate, as in thecase of current art coupling agents. However, contrary to known systems,the other end of the silane is reversibly bound to the acid-functionalfilm former through ionic interactions It is thus hypothesized that themicroscopic interphase region between the substrate and the polymerremains tough and immobile at ordinary temperatures, but is relativelyfluid at the elevated temperatures and high shear rates experiencedduring injection molding.

The present invention therefore relates to a composition comprising:

(I) an acid-functional silane;

(II) an acid-functional film former selected from the group consistingof carboxylated thermoplastic polymers and carboxylated thermoplasticcopolymers; and

(III) a sufficient amount of an ionic compound, having a cation selectedfrom the group consisting of monovalent and divalent metal ions, topartially neutralize the total acid functionality present in saidacid-functional silane (I) and said acid-functional film former (II).

This invention also relates to a composition comprising the abovementioned acid-functional film former (II) and (IV) a sufficient amountof a metal salt of said acid-functional silane (I), having a cationselected from the group consisting of monovalent and divalent metalions, to partially neutralize the acid functionality present in saidacid-functional film former (II).

The present invention further relates to a process for bonding athermoplastic matrix polymer or acid-modified thermoplastic matrixpolymer to a substrate comprising:

(a) treating said substrate with one of the above desribed compositions;and

(b) fusing said matrix polymer to the treated substrate resulting fromstep (a).

DETAILED DESCRIPTION OF THE INVENTION

In a first aspect of the present invention, an ionomeric silane couplingagent composition is prepared by mixing (I) an acid-functional silane,(II) an acid-functional film former and (III) an ionic compound having ametal cation.

The acid-functional silane (I) consists of an acid group covalentlybonded to a trialkoxysilane, trihydroxysilane or silsesquioxanestructure by an organic connecting group. The exact nature of theconnecting group is inconsequential as long as it is inert with respectto the other components of the coupling agent composition. Thus, theacid-functional silane has one of the following structures in itsmolecule, oligomeric siloxane condensation products thereof also beingwithin the scope of the present invention:

    Z--Q--SiO.sub.3/2

    Z--Q--Si(OH).sub.3

    Z--Q--Si(OR''').sub.3

in which Z denotes an acidic group and Q is a divalent organicconnecting group. In the last formula, R''' is selected from methyl,ethyl or propyl radicals. The acidic group Z can be any functional groupderived from a protonated oxy acid of carbon, phosphorous, sulfur,selenium or arsenic. Examples of such acidic groups include sulfonic,selenic, arsenic, phosphoric, phosphonic and carboxylic acidfunctionalities. It is preferred that Z is carboxylic acid (--COOH)functionality

The connecting group Q is preferably a short chain hydrocarbon, such asdimethylene or trimethylene, or an aromatic group, such as phenylene orethylphenylene.

Specific examples of the acid-functional silane (I) include thefollowing structures:

    .sub.3/2 OSi--CH.sub.2 CH.sub.2 CH.sub.2 --COOH

    .sub.3/2 OSi--CH.sub.2 CH.sub.2 CH.sub.2 --O--CH.sub.2 CH(OH)CH.sub.2 --SO.sub.3 H ##STR1##

    .sub.3/2 OSi--CH.sub.2 CH.sub.2 COOH

    (CH.sub.3 O).sub.3 Si--CH.sub.2 CH.sub.2 COOH ##STR2##

The acid-functional film former (II) is selected from carboxylatedthermoplastic polymers or carboxylated thermoplastic copolymers. Thesematerials, many of which are available commercially, are well known inthe art. They are typically formed by copolymerizing a minor portion(usually no more than about 10 mole percent) of a carboxy-functionalmonomer with one or more reactive monomers so as to leave pendant orterminal --COOH groups on the resulting polymer or copolymer. They mayalso be formed by grafting carboxylic acid functionality onto a polymerchain. In general, such carboxylated systems are the result ofaddition-type polymerizations typically free radical polymerizations,but may also be based on carboxylated condensation polymers such aspolyurethanes, polyesters and alkyd resins. Component (II) is preferablyselected from carboxylated polymers of polyethylene,poly(methylmethacrylate), copolymers of ethylene with acrylic ormethacrylic acid and styrene-butadiene copolymers.

The acid-functional film former (II) may be incorporated into thecompositions of the present invention in aqueous or solvent dispersionform. Preferably, it is added as a water emulsion.

The ionic compound (III) is selected from salts, hydroxides or oxides ofmonovalent or divalent metals. When a metal salt is used, it ispreferred that it be a water-soluble organic salt, such as an acetate orformate. Halide salts are considered unsuitable herein, however.Examples of suitable ionic compounds include those having sodium,lithium, zinc, calcium, magnesium or potassium cations. Preferred ioniccompounds are sodium hydroxide and zinc acetate.

In order to form the compositions according to the first aspect of thepresent invention a mixture of components (I), (II) and (III) isprepared. The molar ratio of component (II) to component (I) in thismixture is between about 0.01 and 100 based on the acidic groups oneach. Preferably approximately equal molar quantities are used. Theamount of component (III) employed is such that the total acidfunctionality of components (I) and (II) is at least partiallyneutralized by the metal ion present in component (III). Those skilledin the art will readily determine the optimum degree of neutralizationrequired for a particular system through routine experimentation.Preferably, from about 20% to 80% the acid functionality is neutralizedby the metal (on a molar equivalent basis). Most preferably, from about30% to 60% of the acid functionality is so neutralized.

In a second aspect of the present invention, a ionomeric silane couplingagent is prepared by mixing a metal salt of the acid-functional silane(I) with the acid-functional film former (II) and, optionally, the ioniccompound (III). The skilled artisan will of course recognize that insuch a salt, the metal ion can associate with the oxygens on silicon inaddition to associating with the acidic functionality in (I) and itsstructure is generally not readily determinable. Thus, for example, thestructure would best be represented by a formula such as ##STR3## forthe case of a sodium salt of a sulfonic acid-functional silane, whereinx is sufficient to impart neutrality to the salt.

The acid-functional silanes and their metal salts, described above, arewell known in the art. Examples of such compounds, along with methodsfor their preparation, may be found in U.S. Pat. No. 4,344 860 U.S. Pat.No. 4,370,255, U.S. Pat. No. 4,503,242 and U.S. Pat. No. 3,956,353.

In a second aspect of the compositions of the present invention, thesame metal cations disclosed above in the description of component (III)are employed as the counter ion in (IV), a salt of the acid-functionalsilane (I). The metal cation of such a salt thus serves to neutralizedsome of the acidity associated with component (II) and serves to reducethe amount of component (III) needed. It is also within the scope of thepresent invention to completely eliminate component (III) when theamount of the metal salt of the acid-functional silane suppliessufficient metal ion to partially neutralize the acidity present incomponent (II). The preferred degree of neutralization is the same asdiscussed in connection with the first aspect of the present invention,above.

In order to form the compositions according to the second aspect of thepresent invention, a mixture of components (II), (IV) and optionally,(III) is prepared. Since the degree of neutralization of the acidity isdetermined through routine experimentation, as above the relativeamounts of component (II) and component (IV) is likewise experimentallyobtained, as is the amount of component (III), if any. As before, it ispreferred that from about 20% to 80% the total acid functionality isneutralized by the metal ion (on a molar equivalent basis), 30% to 60%neutralization being most preferred.

The components of the present invention can be dispersed in solventssuch as methanol, ethanol and propylene glycol monomethyl ether. It ispreferred, however, that mixing be carried out in a water dispersion,from which the composition may be applied to a substrate, as describedinfra. The acidfunctional silanes are generally soluble in water butmethods which may also be used to disperse silane coupling agents inwater are described by Plueddemann in U.S. Pat. No. 3,258,477.

The present invention also relates to a process for bonding athermoplastic matrix polymer to a substrate by (a) treating thesubstrate with a composition of the present invention and (b) fusing thethermoplastic matrix polymer to the treated substrate resulting fromstep (a).

In a first embodiment of the process of the present invention, thematrix polymer is selected from thermoplastic polymers or copolymerssuch as polyethylene, nylon, styrenebutadiene copolymers, polyolefincopolymers, polyesters and poly(vinyl chloride).

The choice of a given matrix polymer or copolymer dictates the type ofacid-functional film former (II) to be used in the coupling agentcomposition inasmuch as these two materials must be compatible (i.e.,they do not phase separate). Thus, for example, when the matrix polymeris polyethylene, the acid-functional film former is preferably acarboxylated polyethylene.

In the above process, a substrate is first treated with one of thepreviously described compositions of the present invention according tomethods well established in the art. The silane coupling agents may beapplied to substrates by dipping, spraying, dry blending methods, suchas tumbling with a mineral filler in a container, or by mechanicalmixing with a filler, followed by drying in air at 100° to 175° C.Preferably, the ionomeric silane coupling agent is deposited onto thesurface of the substrate from a water dispersion and the treatedsubstrate dried at temperatures between 100° and 175° C.

The treated substrate may then be bonded to the matrix polymer by fusingthe latter onto the former at a temperature sufficient to impartfluidity to the polymer (e.g., above the melt point in the case of acrystalline polymer).

Substrates contemplated herein can be fillers which are typically usedto extend or reinforce the above mentioned thermoplastic matrixpolymers. They are inorganic materials which may be of natural orsynthetic origin, but have a common feature in that their surfacescontain hydroxyl functionality to a greater or lesser extent. Notablewithin this general category of fillers are the siliceous materials suchas glass fiber, precipitated silica, ground quartz, aluminum silicate,zirconium silicate, calcium silicate, glass micro beads, mica, asbestos,clay, vitreous enamels and ceramics. Other examples of suitable fillersinclude alumina, silicon carbide, silicon whiskers, metals and metaloxides.

In addition to the filler, other components, such as catalysts,pigments, stabilizers and antioxidants may be included in a typicalfilled polymer formulation. These formulations may be molded intodesired shapes by, e.g., compression or injection molding. As notedabove, the coupling agents of the present invention are of particularadvantage in treating reinforcing fillers, such as glass fibers, for usein compositions for injection molding.

The substrate may also consist of a bulk material, wherein the couplingagents of the present invention are used to prime the surfaces thereof.Examples of such substrates include metals, metal oxides, glass, micacomposites, asbestos composites, fired clay, vitreous enamel, siliconcarbide, alumina and ceramics, inter alia. Methods for using silanecoupling agents as primers are well known in the art. Typically, thesurface of a substrate is wetted by the coupling agent by dipping,brushing, spraying, or wiping, for example. As before the silane may beapplied from solution or dispersion, the preferred method beingapplication from aqueous solution or dispersion at about a 5-20% (byweight) concentration. After application, the primed surface is usuallydried to remove any solvent or water employed. The primed surface ofthis invention forms water-resistant bonds to the matrix polymer when itis fused thereon.

In a second embodiment of the process of the present invention, thematrix polymer is selected from thermoplastic acid-modified polymers orcopolymers. These materials are also well known in the art and aresubstantially identical to the matrix polymers described above wherein aminor portion of acid functionality has been copolymerized into the mainpolymer chain or grafted thereto. Thus, the carboxylated polymers usedas component (II), supra, form one class of such acid-modified systems.In this case, however, they may also take the form of bulk polymers.

Additionally, the acid-modified polymer may be an ionomer. In this case,the ionic content of the polymer should be taken into account indetermining the proper degree of neutralization of the acidfunctionality of components (I) and (II) of the compositions of thepresent invention.

In the second embodiment of the process of the present invention, it isfurther contemplated that a minor portion (e.g., from about 1 to 10weight percent) of an acid-modified polymer may be blended with acompatible unmodified matrix polymer.

It has also been found that, when the matrix polymer is selected fromacid-modified polymers or the above mentioned blend of acid-modifiedpolymer and unmodified polymer, the need for the acid-functional filmformer of the present invention is reduced or completely eliminated.Thus, this invention also relates to a process for bonding such anacid-modified polymer or blend to a substrate by (a) treating thesubstrate with either a combination of components (I) and (III) or withcomponent (IV) and, as before, (b) fusing the thermoplastic matrixpolymer to the treated substrate resulting from step (a). In eithercase, the principle of partial neutralization, outlined above, againapplies.

EXAMPLES

The following examples are offered for the purpose of illustration andshould not be construed as limiting the claimed invention.

Metal salts of acid-functional silanes used in the examples included thestructures: ##STR4##

Other ingredients used herein were:

PLEXAR-6 is a carboxylated polyethylene (Chemplex Co., Rolling Meadows,Ill.).

PRIMACOR 4983 is an aqueous emulsion of a polyethylene-acrylic acidcopolymer (Dow Chemical Co., Midland, Mich.).

EXAMPLES 1-3

A primer solution of SILANE C (i.e. component IV of the presentinvention) was prepared by diluting one mole of this compound with waterto form 1000 grams of solution. The primer solution was applied to apre-cleaned glass microscope slide by wiping with a paper tissue andallowing the coating to dry at 100° C. for 15 minutes. A five mil thickfilm of PLEXAR-6 was then fused onto the primed slide by pressing thecomposite at about 200° C. for one minute. A control, using an unprimedslide, was similarly processed. Initial adhesion of the acid-modifiedpolymer to the glass surface was determined by prying or scraping thefilms from the glass slides using a razor blade.

The slides were then submerged in water at room temperature and theadhesion of the polymer to the primed glass was monitored. Results arepresented in Table 1, wherein the following rating scheme pertains:

    ______________________________________    Rating      Observation    ______________________________________    nil         Fell off (Dry) or Floated free of                glass slide (Wet)                (adhesive failure).    fair        Could be removed in one piece with                razor blade (adhesive failure).    good        Could be pried off in pieces (adhesive                and cohesive failure).    excellent   Could not be removed from glass                (cohesive failure).    ______________________________________

Time to failure, reported in Table 1, is defined as the point at whichthe adhesion rating dropped below "good" or fell off completely. As canbe seen from Table 1, the primed system took considerably longer to failthan the control, (Comparison) Example 1.

                  TABLE 1    ______________________________________            Silane  Initial Adhesion            Primer  Rating      Time to Failure    ______________________________________    (Comparative)    Example 1 None      fair        1 hour (nil rating)    Example 2 SILANE C  excellent   2 days (fair                                    rating)    ______________________________________

EXAMPLE 3-7

Water solutions of SILANE A, SILANE B and SILANE C (one mole per 1000grams of solution) were mixed with equal weights of PRIMACOR 4983emulsion (i.e., the film former of the present invention) which had beendiluted with water so as to contain 1.2 moles of carboxylic acid groupsper 1000 grams of solution. It was calculated that these mixtures had adegree of neutralization of 40% based on the carboxylic acidfunctionality introduced by the film former. These mixtures were used toprime glass slides, as described above, and the coatings dried at 100°C. for 15 minutes. High density polyethylene was pressed onto each slideat a temperature of 250° C. and the composites were tested as before.Initial adhesion and time to failure are reported in Table 2.(Comparative) Example 3 shows results for an unprimed slide and(Comparative) Example 4 shows results for a slide primed only with thePRIMACOR 4983 film former.

                  TABLE 2    ______________________________________                     Initial                     Adhesion           Primer    Rating    Time to Failure    ______________________________________    Comparative)             None        poor      1 hour (nil rating)    Example 3    Comparative)             PRIMACOR    excellent 2 hours (nil rating)    Example 4             4983    Example 5             SILANE A/   excellent 4 days (fair rating)             PRIMACOR    Example 6             SILANE B/   excellent 1 day (fair rating)             PRIMACOR    Example 7             SILANE C/   excellent 2 days             PRIMACOR    ______________________________________

It can be seen from table 2 that the compositions of the presentinvention retain their excellent adhesion much longer than the controlswhen exposed to a water environment.

I claim:
 1. A process for bonding a thermoplastic acid-modified matrixpolymer to a substrate comprising:(a) treating said substrate with acomposition comprising an acid-functional silane and an ionic compoundhaving a cation selected from the group consisting of monovalent anddivalent metal ions; and (b) fusing said thermoplastic acid-modifiedmatrix polymer to the treated substrate resulting from step (a), saidmetal cation being present to an extent sufficient to partiallyneutralize the acid functionality supplied by said acid-functionalsilane and said thermoplastic acid-modified polymer matrix.
 2. A processaccording to claim 1, wherein the acid functionality of saidacid-functional silane is carboxylic.
 3. A process according to claim 2,wherein said ionic compound is present to an extent sufficient toneutralize about 20 to 80 percent the total acid functionality of saidacid-functional silane and said thermoplastic acid-modified polymer. 4.A process according to claim 3, wherein said metal cation is is selectedfrom the group consisting of sodium and zinc ions.
 5. A process forbonding a thermoplastic matrix polymer to a substrate comprising:(a)treating said substrate with a composition comprising (I) anacid-functional silane, (II) an acid-functional film former selectedfrom the group consisting of carboxylated thermoplastic polymers andcarboxylated thermoplastic copolymers and (III) an ionic compound havinga cation selected from the group consisting of monovalent and divalentmetal ions; and (b) fusing said thermoplastic matrix polymer to thetreated substrate resulting from step (a), said metal cation beingpresent to an extent sufficient to partially neutralize the acidfunctionality supplied by said acid-functional silane and saidacid-functional film former.
 6. A process according to claim 5, whereinthe acid functionality of said acid-functional silane is carboxylic. 7.A process according to claim 6, wherein said ionic compound is presentto an extent sufficient to neutralize about 20 to 80 percent the totalacid functionality of components (I) and (II) on a molar equivalentbasis.
 8. A process according to claim 7, wherein said metal cation ofsaid ionic compound is selected from the group consisting of sodium andzinc ions.
 9. A process for bonding a thermoplastic matrix polymer to asubstrate comprising:(a) treating said substrate with a compositioncomprising an acid-functional film former selected from the groupconsisting of carboxylated thermoplastic polymers and carboxylatedthermoplastic copolymers and a sufficient amount of a metal salt of anacid-functional silane, having a cation selected from the groupconsisting of monovalent and divalent metal ions to partially neutralizethe acid functionality present in said acid-functional film former; and(b) fusing said thermoplastic matrix polymer to the treated substrateresulting from step (a), said metal cation being present to an extentsufficient to partially neutralize the acid functionality supplied bysaid acid-functional film former.
 10. A process according to claim 9,wherein said metal cation is present to an extent sufficient toneutralize about 20 to 80 percent the total acid functionality of saidacidfunctional film former
 11. A process according to claim 10, whereinsaid metal cation is selected from the group consisting of sodium andzinc ions.
 12. An article of manufacture prepared according to theprocess of claim
 1. 13. An article of manufacture prepared according tothe process of claim
 5. 14. An article of manufacture prepared accordingto the process of claim 9.